MEDIUM: Allow suppression of email alerts by log level

MEDIUM: Allow suppression of email alerts by log level

This patch adds a new option which allows configuration of the maximum
log level of messages for which email alerts will be sent.

The default is alert which is more restrictive than
the current code which sends email alerts for all priorities.
That behaviour may be configured using the new configuration
option to set the maximum level to notice or greater.

	email-alert level notice

Signed-off-by: Simon Horman <horms@verge.net.au>